High temperature compact thermoelectric module with gapless eggcrate

ABSTRACT

A high-temperature thermoelectric module. The module includes a two part molded egg-crate for holding in place and providing insulation and electrical connections for a number of thermoelectric n-legs and p-legs. A first part of the egg-crate is comprised of a ceramic material capable of operation at temperatures in excess of 600° C. and a second part comprised of a thermoplastic material having very low thermal conductivity. In preferred embodiments the high temperature ceramic is zirconia and the thermoplastic material is DuPont Zenite. The thermoelectric legs are also comprised of high-temperature and low-temperature material. In preferred embodiments the high temperature thermoelectric material is lead telluride and the low temperature material is bismuth telluride. In preferred embodiments metal felt spacers are provided in each leg to maintain proper electrical contacts notwithstanding substantial temperature variations. Preferably the module is sealed in an inert gas filled insulating capsule.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of Provisional Patent Application, Ser. No. 61/137,206, filed Jul. 17, 2008.

FIELD OF THE INVENTION

The present invention relates to thermoelectric modules and especially to high temperature thermoelectric modules.

BACKGROUND OF THE INVENTION Thermoelectric Materials

The Seebeck coefficient of a thermoelectric material is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference of 1 K exists between those points.

The figure-of-merit of a thermoelectric material is defined as:

${Z = \frac{\alpha^{2}\sigma}{\lambda}},$

where α is the Seebeck coefficient of the material (measured in microvolts/K), σ is the electrical conductivity of the material and λ is the total thermal conductivity of the material.

A large number of semiconductor materials were being investigated by the late 1950's and early 1960's, several of which emerged with Z values significantly higher than in metals or metal alloys. No single compound semiconductor evolved that exhibited a uniform high figure-of-merit over a wide temperature range, so research focused on developing materials with high figure-of-merit values over relatively narrow temperature ranges. Of the great number of materials investigated, those based on bismuth telluride, lead telluride and silicon-germanium alloys emerged as the best for operating in various temperature ranges. Much research has been done to improve the thermoelectric properties of the above three thermoelectric materials. For example n-type bismuth telluride, Bi₂Te₃ typically contains 5 to 15 percent Bi₂Se3 and p-type Bi₂Te₃ typically contains 80 Mol percent Sb₂Te₃. Lead telluride is typically doped with sodium for P type and Pbl₂ iodine for N type.

Thermoelectric Modules

Electric power generating thermoelectric modules are well known. These modules produce electricity directly from a temperature differential utilizing the thermoelectric effect. The effect is that a voltage differential of a few millivolts is created in the presence of a temperature difference at the two junctions of p-type thermoelectric semiconductor elements and n-type thermoelectric semiconductor elements. These thermoelectric elements are called n-legs and p-legs. Since the voltage differential is small, many of these elements (such as about 100 elements) are typically positioned in parallel between a hot surface and a cold surface and are connected electrically in series to produce potentials of a few volts.

Hi-Z Prior Art Bismuth Telluride Molded Egg-Crate Modules

For example Hi-Z Technology, Inc. offers a Model HZ-14 thermoelectric bismuth telluride thermoelectric module designed to produce about 14 watts at a load potential of 1.66 volts with a 200° C. temperature differential. Its open circuit potential is 3.5 volts. The module contains 49 n-legs and 49 p-legs connected electrically in series. It is a 0.5 cm thick square module with 6.27 cm sides. The legs are p-type and n-type bismuth telluride semiconductor legs and are positioned in an egg-crate type structure that insulates the legs from each other except where they are intentionally connected in series at the top and bottom surfaces of the module. That egg-crate structure which has spaces for 98 legs is described in U.S. Pat. No. 5,875,098 which is hereby incorporated herein by reference. The egg-crate is injection molded in a process described in detail in the patent. This egg-crate has greatly reduced the fabrication cost of these modules and improved performance for reasons explained in the patent. FIG. 1 is a drawing of the egg-crate and FIG. 2 is a cross sectional drawing of a portion of the egg-crate showing how the p-legs and n-legs are connected in series in the egg-crate. The curved arrows e show the direction of electron flow through bottom conductors 2, n-legs 4, top conductors 6, and p-legs 8 in this portion 10 of the module. Insulating walls 14 keep the electrons flowing in the desired series circuit. Other Bi₂Te₃ thermoelectric modules that are available at Hi-Z are designed to produce 2.5 watts, 9 watts, 14 watts and 20 watts at the 200° C. temperature differential.

Temperature Limitations

The egg-crates for the above described Bi₂Te₃ modules are injection molded using a thermoplastic supplied by Dupont under the trade name “Zenite”. Zenite melts at a temperature of about 350° C. The thermoelectric properties of Bi₂Te₃ peak at about 100° C. and are greatly reduced at about 250° C. For both of these reasons, uses of these modules are limited to applications where the hot side temperatures are lower than about 250° C.

Lead Telluride Modules

Lead telluride thermoelectric modules are also known in the prior art. A prior art example is the PbTe thermoelectric module described in U.S. Pat. No. 4,611,089 issued many years ago to two of the present inventors. This patent is hereby incorporated herein by reference. That module utilized lead telluride thermoelectric alloys with an excess of lead for the n-legs and lead telluride with an excess of tellurium for the p-legs. The thermoelectric properties of lead telluride thermoelectric alloys peak in the range of about 425° C. The egg-crate for the module described in the above patent was fabricated using a technique similar to the technique used many years ago for making chicken egg crates from cardboard spacers. For the thermoelectric egg-crate the spacers were mica which was selected for its electrical insulating properties at high temperatures. Mica, however, is marginal in strength and cracks easily. In addition the walls of the egg-crate made from the mica spacers all had gaps at the intersections of the walls that could lead to short circuits. A more rugged high temperature egg-crate with gapless walls is needed.

FIG. 3 is a drawing from the U.S. Pat. No. 4,611,089 patent showing a blow-up of that module. The egg-crate included a first set of parallel spacers 46 a to 46 k and a second set of spacers 48 a to 48 i. The n-legs are shown at 52 and the p-legs are shown at 54. The module included hot side conductors 56 and cold side conductors 58 to connect the legs in series as in the Bi₂Te₃ module described above.

That lead telluride module was suited for operation in temperature ranges in excess of 500° C. But the cost of fabrication of this prior art module is greatly in excess of the BiTe module described above. Also, after a period of operation of about 1000 hours some evaporation of the p-legs and the n-legs at the hot side would produce cross contamination of all of the legs which would result in degraded performance.

What is needed is a low cost, compact, high-temperature thermoelectric module with gapless walls designed for operation at hot side temperatures in excess of 500° C. preferably with thermoelectric properties substantially in excess of prior art high-temperature thermoelectric modules.

SUMMARY OF THE INVENTION

The present invention provides a high-temperature thermoelectric module. The module includes a two-part molded egg-crate for holding in place and providing insulation and electrical connections for a number of thermoelectric n-legs and p-legs. A first part of the egg-crate is comprised of a ceramic material capable of operation at temperatures in excess of 500° C. and a second part comprised of a thermoplastic material having very low thermal conductivity. In preferred embodiments the high temperature ceramic is zirconium oxide and the thermoplastic material is a DuPont Zenite available from DuPont in the form of a liquid crystal polymer resin. The thermoelectric legs of preferred embodiments are also comprised of high-temperature and low-temperature material. In preferred embodiments the high-temperature thermoelectric material is lead telluride and the low-temperature material is bismuth telluride thermoelectric alloys described in the background section. In preferred embodiments metal felt spacers are provided in each leg to maintain proper electrical contacts notwithstanding substantial temperature variations. Preferably the module is sealed in an insulating capsule.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a prior art egg-crate for a thermoelectric module.

FIG. 2 is a drawing of a portion of a module with the FIG. 1 egg-crate.

FIG. 3 is a prior art blown-up drawing of a prior art lead telluride thermoelectric module.

FIGS. 4 and 4A are drawings showing important features of a preferred embodiment of the present invention.

FIG. 5 is a drawing showing an application of the preferred embodiment used to generate electricity from the exhaust gas of a truck.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred Embodiment

A first preferred embodiment of the present invention can be described by reference to FIGS. 4 and 4A. The drawing is similar to the FIG. 3 drawing, but the module is greatly improved from the module described in U.S. Pat. No. 4,611,089.

The Egg-Crate

Egg-crate 70 is injection molded using a technique similar to that described in U.S. Pat. No. 5,875,098. However, the molding process in substantially more complicated. The egg-crate comes in two molded together sections. It includes an upper section (which will lie adjacent to a hot side) molded from stabilized zirconium oxide (ZrO₂). ZrO₂ has a very high melting point of 2715° C. and a very low thermal conductivity for an oxide. The egg-crate also includes a lower side (which will lie adjacent to a cold side) molded from Zenite Model 7130 available from Dow Chemical that has a melting point of 350° C. but very low thermal conductivity.

The ZrO₂ portion of the egg-crate is fabricated by injection molding of the ZrO₂ powders with two different binder materials. Some of the binder material is removed by leaching prior to sintering. The ZrO₂ portion is then sintered to remove the second binder and produce a part with good density and high temperature strength. The ZrO₂ portion will typically shrink about 20 percent during sintering. This sintered section is then placed in a second mold and a subsequent injection molding of the Zenite portion of the egg-crate is then performed, thereby bonding the Zenite to the ZrO₂. While the thermal conductivity of the ZrO₂ is among the lowest of any known oxide, its thermal conductivity of 2 W/mK is much higher than the thermal conductivity of Zenite which is 0.27 W/mK. An objective of the present invention is to minimize any loss of heat through the egg-crate material. Also the Zenite is flexible and will allow the two-section egg-crate to endure significant rough handling. The mica of the prior art patent is a relatively weak material that cracks easily. FIG. 4A shows a preferred technique for assuring a good bond between the ZrO₂ portion and the Zenite portion. A tab shown at 30 is molded at the bottom of the ZrO₂ walls 32. This increases the bonding surface between the ZrO₂ portion 32 and the Zenite portion 34 of the egg-crate walls.

Two Types of Thermoelectric Legs

The PbTe/Bi₂Te₃ thermoelectric legs of this preferred embodiment are segmented as shown at 72 and 74 in FIG. 4. The top portion 72 a of n-leg 72 is comprised of lead telluride thermoelectric material and the bottom portion 72 b is comprised of bismuth telluride thermoelectric material. The top lead telluride portion 72 a is doped with 0.055 Mol percent PbI₂ to create high temperature n-type material. The bottom bismuth telluride portion 72 b is doped with 0.1 Mol percent iodoform (CHI₃) to create the lower temperature n-type material. The top portion 74 a of p-leg 74 is comprised of lead telluride material and the bottom portion 74 b is comprised of bismuth telluride material. The top lead telluride portion 74 a is doped with 1.0 atomic percent Na to create high temperature p-type material. The bottom bismuth telluride portion 74 b is doped with 0.1 parts per million Pb to create lower temperature p-type material.

Other Module Component

Egg-crate 70 contains spaces for 80 legs, 40 n-legs and 40 p-legs. The components of these legs are shown blown-up in FIG. 4. At the top is hot conductor 76 comprised of iron metal. Below hot conductor 76 in the p-leg 74 is p contact 78 which is an approximately 0.040 inch thick graphite spacer and is needed to prevent interaction of the Fe hot shoe and the p-type PbTe. The lead telluride portion 74 a of segmented p-leg 74 is in contact with contact 78. The lead telluride portion 72 a of segmented n-leg 72 is in direct contact with iron conductor 76. Below both legs as shown in FIG. 4 is the Pb compatibility foil 80 which prevents any contamination from cold conductor 82 which is preferably made from copper sheet material about 0.010 inch thick. Below cold conductor 82 in both legs is insulating sheet of aluminum oxide 84 and below insulating sheet 84 is fiber metal felt material 86 (discussed in more detail below) which is made from copper or bronze felt material.

Technique to Prevent The Evaporation and Contamination

As indicated in the background section life testing by Applicants of PbTe modules has shown that some degradation of the module occurs after approximately 1,000 hours of operation. Applicants have discovered that the degradation can be attributed to “cross-talk” between the n-legs and the p-legs near the hot junction caused by evaporation of tellurium from the p-leg contaminating the n-leg. (As explained in the background section an excess of lead in the n-leg is what provides the n-leg with its thermoelectric doping properties.) The problem is prevented in this embodiment with two techniques: First, as shown in FIG. 4 the egg-crate walls separating the n-legs from the p-legs are extended to contact the hot conductor 76 so that tellurium vapor is restrained from migrating to the n-leg. Another technique used by applicants in this embodiment is to add a thin layer of PbSnMnTe at the top (hot side) of the p-legs (not shown in the drawings). Applicants have determined that elemental tellurium exhibits little or no evaporation from PbSnMnTe. While the PbSnMnTe material does not have as good thermoelectric properties as PbTe, the amount used is small, only 0.020 inch long out of 0.450 inch overall length. The PbSnMnTe segments will be cold pressed and sintered with the p-type PbTe. In some embodiments the PbSnMnTe material may be substituted for the hot portion of the p-legs.

Good Thermal and Electrical Conductivity

Compliant Metal Parts

The thermoelectric module of this preferred embodiment will typically be placed between a hot surface of about 600° C. and a cold surface of about 50° C. In many applications these temperatures may vary widely with temperature differentials swinging from 0° C. to 550° C. Therefore the module and its components should be able to withstand these temperatures and these changes in temperature which will produce huge stresses on the module and its components. This embodiment is designed to meet these challenges.

This preferred embodiment includes at the bottom of each leg at the cold side a fiber metal felt pad 86 comprised of copper or bronze wool. These materials provide good thermal conductivity and are able to deform when the module is placed in compression between the heat source and heat sink. These materials are resilient to respond to thermal cycling and to the inevitable warping that the module will undergo because of the thermal gradient imposed across it. These compliant materials are also able to cushion the brittle lead telluride yet maintain intimate contact between it and the heat sink. The fiber metal felt pads can be impregnated with an elastomer such as silicone rubber to reduce the risk of creep and to add deflection and compliance. Silicone rubber can operate at temperatures up to 300° C.

The n and p type PbTe legs are creeping or pushing up towards the Fe hot shoe. A load of 1,000 psi is initially used and this load can be reduced to 500 psi after the module is operated for approximately 100 hours and proper seating of the module is obtained. After this time a lower load of 50 to 100 psi can be used to maintain the low contact resistance joints.

Overall Module Design

The module is specifically designed to endure considerable thermal cycling or steady state behavior. The hot and cold side joints are free to slide and relieve thermal stresses. The module needs to be held in compression at approximately 50 to 100 psi after it reaches its design operating temperatures.

Alternate Bulk Alloys

Lead telluride based alloys have been used since the 1960s and the alloys and recommended doping levels are well documented in the prior art literature. Their thermoelectric properties versus temperature are given in many publications such as Chapter 10, “Lead Telluride Alloys and Junctions” of Thermoelectric Materials and Devices, Cadoff and Miller, published by Reinhold Publishing Corporation of New York.

In the past four years newer PbTe based alloys have evolved that have better properties than the conventional PbTe based alloys noted above. For example a Jul. 25, 2008 article in Science Daily reported on a lead telluride material developed at Ohio State University having substantial improvements in efficiency over prior art lead telluride materials. This new material is doped with thallium instead of sodium. The article suggests that efficiency of the new material may be twice the efficiency of prior art lead telluride. Applicants are seeking to prepare bulk lead telluride thermoelectric material with a finer grain size than has previously been achieved. Fine grain size is expected to lower the thermal conductivity of the material without significant impact on resistivity or Seebeck coefficient, thus raising its ZT and efficiency. In previous attempts others have made to produce a fine-grained PbTe, the grain size was observed to coarsen rapidly, even near room temperature, so the benefit of small grain size could not be retained. In this study, Applicants seek to preserve a fine grain structure by additions of very fine alumina powder, which is expected to produce a grain boundary pinning effect, thus stabilizing the fine grain size. Applicants' recent results indicate that the PbTe grain size can indeed be held below 2 μm, even with processing at 800° C.

Lead Telluride Only Modules

A second preferred embodiment of the present invention is just like the first preferred embodiment except the entire legs are comprised of lead telluride thermoelectric alloys. Preferably, the lead telluride alloy is one of the newer very high efficient alloys.

Generator Design Using PbTe Type Modules of the Preferred Embodiment

The high-temperature module of the preferred embodiment requires encapsulation to prevent oxidation of the n and p alloys with an accompanying decrease in thermoelectric properties. An example of encapsulating PbTe modules would be the 1 kW generator for diesel trucks shown at 16 in FIG. 5. In this example the generator is attached to a 5 inch diameter exhaust pipe 18. Lead telluride thermoelectric modules 20 are mounted on a support structure 22 which is machined to form an octagon. The inside surface is generally round with fins (not shown) which protrude into the gas stream to provide a greater heat transfer area. The basic design is similar to the design described in U.S. Pat. No. 5,625,245 which is hereby incorporated herein by reference.

The casting of the support structure has two flanges 24 and 26, one large and one small which are perpendicular to the main part of the support structure. The large flange 24 is about 10 inches in diameter while the small flange 26 is about 8 inches in diameter.

The large flange contains feed-throughs for both the two electrical connections and four water connections. The two electric feed-throughs 28 are electrically isolated with alumina insulators from the support structure. Both the large and small flange will contain a weld preparation so a metal dust cover 30 can be welded in position.

The four water feed-throughs 32 consists of one inch diameter tubes that are welded into the flange. The inside portion of the water tubes are welded to a wire reinforced metal bellows hose with the other end connected to the heat sink by a compression fitting or a stainless to aluminum bimetallic joint. Two of the tubes are inlets and the other two tubes are outlets for the cooling water.

Once the generator is assembled and the flanges between the support structure and the dust cover are welded as shown at 34, the interior volume will be evacuated and back filled with an inert gas such as Argon through a small ⅜ inch diameter tube 36 in the dust cover to about 75% of one atmosphere when at normal room temperature (˜20√ c.). Once filled, the fill tube will be pinched off and welded.

In a preferred embodiment nine thermoelectric modules 20 of the first preferred embodiment are mounted on each of the eight sides of the hexagonal structure for a total of 96 modules. Applicants estimate a total electrical output of about 1.1 kilowatts. This estimate is based on prior performance with the structure described in the U.S. Pat. No. 5,625,245 patent, utilizing modules of the first preferred embodiment and assuming an exhaust hot side temperature of about 550° C. and cold side cooling water temperature of about 100° C.

Variations

While the above description contains many specificities, the reader should not construe these as limitations on the scope of the invention, but merely as exemplifications of preferred embodiments thereof. For example:

Other High Temperature Thermoelectric Alloys

Some of the other thermoelectric alloys that are attractive over high-temperature ranges are:

-   -   20% Si-80% Ge, LaTe_(1.4)     -   Zintl, (Yb₁₄MnSb₁₁)     -   TAGS (AgSbTe₂)_(0.15)(GeTe)_(0.85)     -   The skutterudites such as CoSb4     -   The half-Huesler alloys     -   The LAST and FAST alloys of Michigan State University

All of these bulk alloys and others under development can be used in the new ZrO2/Zenite egg-crate design shown in FIG. 4.

Thin Film Quantum Well Modules

The egg-crate of the present invention could be utilized with thin film quantum well thermoelectric p and n legs of the type described in detail in U.S. Pat. No. 5,550,387 which is incorporated herein by reference. That patent describes n and p thermoelectric legs that are fabricated using alternating layers 10 nanometers thick of Si/Si_(0.8)Ge_(0.2) layers grown on silicon substrates. In applications with temperatures above 500° C. these legs would be used on the cold side. That patent and U.S. Pat. No. 6,828,579 also disclose high temperature lattices comprised of thin layers of B₄C/B₉C and Si/SiC can also be operated at very high temperatures up to about 1100° C. Details for fabricating B₄C/B₉C thermoelectric legs are provided in U.S. Pat. No. 6,828,579 (assigned to Applicants' employer) which is also incorporated herein by reference. See especially Col. 3 where high temperature performance is discussed. These B₄C/B₉C and Si/SiC materials could also be used alone to make thermoelectric legs which could be used in the egg-crate of the present invention or on the hot side of the legs along with bismuth telluride, lead telluride or quantum well Si/SiGe for the cold side.

A large number of 10 nm quantum well layers are built up on a compatible substrate that has a low thermal conductivity to produce quantum well thermoelectric film. Kapton is a good substitute candidate if the temperature is not too high. For higher temperature operation silicon is a preferred choice of substrate material as described in Col. 11 of U.S. Pat. No. 6,828,579. Other substrate materials are discussed in Col. 7. A good substrate material not disclosed in the patent is porous silicon. Porous silicon can survive very high temperatures and has extremely low thermal conductivity. The pores can be produced in silicon film such as 5 micron thick film from one side to extend to within a fraction of a micron of the other side. The pores can be produced either before the thermoelectric layers are laid down or after they are laid down.

The quantum well thermoelectric film is cut and combined to make n and p type legs of the appropriate size and each leg is loaded into one opening of the FIG. 4 egg-crate. Before loading the hot and cold ends of the N and P legs are metallized to yield a low contact resistance on the cold side. Electrical contacting materials of Pb foil and Cu connecting straps are then assembled followed by the Al₂O₃ spacer and Cu felt. The module is then turned over and the graphite piece is placed against the P leg, followed by the iron conductor strap which contacts the graphite and the N type PbTe.

Other Fabrication Techniques

Cyano Glue

A technique that can simplify cold side lay-up involves the use of cyano acrylate glue (called super glue) which can be used to temporarily hold the parts in position during lay-up but is burned off at a seating step after the thermoelectric legs are assembled in the egg-crate.

Other Compliant Members

Other compliant members could be substituted for the copper felt discussed above. These include woven steel felt with a boron nitride slurry to enhance thermal conductivity. A thin Beliville spring could be used. Boron nitride powder could be stirred into the spring material before the spring is cast.

Other Interfaces

Instead the tab-socket technique, other techniques for joining the hot and cold portions of the egg-crate could be used, such as a roughen surface of the ceramic portion.

Segmented Legs

The PbTe portions of the p-legs could be cold pressed and sintered separately from the BiTe portions. When they are subject to hot operating conditions they will diffusion bond. The same applies to the n-legs.

Hot Pressing of the Legs

Another option is to hot press the thermoelectric materials in bulk then slice and dice them into legs.

Other Crate Designs

Aerogel Crate

In one variation, an aerogel material is used to fill all unoccupied spaces in the egg-crate. A module assembly will be sent to Aspen Aerogels. They will immerse it in silica sol immediately after dropping the pH of the sol. The sol converts to silica gel over the next few days. It is then subjected to a supercritical drying process of tightly controlled temperature and pressure condition in a bath of supercritical liquid CO₂. This process removes all the water in the gel and replaces it with gaseous CO₂. The Aerogel serves multiple functions: (1) helping to hold the module together, (2) reducing the sublimation rate of the PbTe and (3) providing thermal and electrical insulation around the legs.

In another variation, a portion of the egg-crate could be made of non-woven refractory oxide fiber material, possibly with a fugitive polymer binder, and having the consistency of stiff paper or card stock is used. After assembly, the binder is burned away, leaving a porous fiber structure that is them infiltrated with Aerogel. The fiber reinforcement of the aerogel gives added strength and toughness.

Those skilled in the art will envision many other possible variations within its scope. Accordingly, the reader is requested to determine the scope of the invention by the appended claims and their legal equivalents, and not by the examples which have been given. 

1. A high-temperature thermoelectric module comprising: A. a two-part molded egg-crate for holding in place and providing insulation and electrical connections for a number of thermoelectric n-legs and p-legs, wherein said egg-crate is comprised of: 1) a hot side part comprised of a ceramic material capable of operation at temperatures in excess of 500° C. and 2) a cold side part comprised of a thermoplastic material having very low thermal conductivity. B. a plurality of high-temperature thermoelectric n-legs and p-legs positioned in said egg-crate, at least a portion of which are electrically connected in series.
 2. The thermoelectric module as in claim 1 wherein said ceramic material is stabilized zirconium oxide and said thermoplastic material is in the form of a liquid crystal polymer resin.
 3. The thermoelectric module as in claim 2 wherein the cold side part and the hot side part are joined together at a tab and socket junction.
 4. The thermoelectric module as in claim 3 wherein the module defines at least four outside walls and a large number of inside walls and the tab and socket junction includes tabs and sockets in the outside walls.
 5. The thermoelectric module as in claim 4 wherein the tab and socket junction also includes tabs and sockets in at least a plurality of the inside walls.
 6. The thermoelectric module as in claim 1 wherein the thermoelectric p-legs and n-legs are comprised of a lead telluride thermoelectric alloy.
 7. The thermoelectric module as in claim 1 wherein the thermoelectric p-legs and n-legs are segmented legs, comprised of a high-temperature material and a low-temperature material.
 8. The thermoelectric module as in claim 7 wherein high-temperature thermoelectric material is a lead telluride thermoelectric alloy and the low-temperature material is a bismuth telluride thermoelectric alloy.
 9. The thermoelectric module as in claim 1 wherein metal felt spacers are provided in each leg to maintain proper electrical contacts notwithstanding substantial temperature variations.
 10. The thermoelectric module as in claim 9 wherein the metal felt spacers are impregnated with an elastomer.
 11. The thermoelectric module as in claim 10 wherein the elastomer is silicon rubber.
 12. The thermoelectric module as in claim 1 wherein the module is sealed in an insulating capsule.
 13. The thermoelectric module as in claim 1 wherein the module is combined with other similar modules to provide a thermoelectric generator.
 14. The thermoelectric module as in claim 13 wherein the thermoelectric generator is adapted to provide electric power from the waste heat of a motor vehicle.
 15. The thermoelectric module as in claim 1 wherein said n-legs are electrically connected to said p-legs at the hot side of the module with a lead telluride compatible hot conductor element.
 16. The thermoelectric module as in claim 15 wherein the compatible hot conductor element is comprised of iron.
 17. The thermoelectric module as in claim 15 wherein the egg-crate walls separating the n-legs from the p-legs are adapted to contact the hot conductor so that tellurium vapor is restrained from migrating to the n-leg.
 18. The thermoelectric module as in claim 7 wherein the p-legs comprise a thin layer of PbSnMnTe or SnTe at their hot sides. 